Hydraulic circuits form the basis for most vehicle braking systems, especially automotive vehicle braking systems. Hydraulic power is particularly suited to such braking systems since it converts fluid pressure into mechanical motion and allows the source of the hydraulic pressure to be positioned remotely from the cylinders which effect the braking action. Such automotive braking systems are generally hydraulic throughout, consisting of an actuator, such as a brake pedal, a reservoir of fluid such as a master cylinder, whose pressure is responsive to pressure applied by the actuator, and means for converting the hydraulic pressure to braking force, such as brake cylinders. In standard systems of this type, braking pressure is achieved mechanically, utilizing the force of the depression of the brake pedal by the driver (usually accompanied by a vacuum boost) to increase the fluid pressure in the master cylinder, which is then transmitted through fluid lines to the cylinders which operate calipers or shoes, thereby forcing the calipers or shoes against the rotors or drums, respectively, to effect braking action.
Antilock braking systems ("ABS") are frequently incorporated into vehicle hydraulic braking systems, such as the aforementioned system, in order to prevent vehicle skidding during "panic" braking events or on wet or snow-covered pavement. Vehicle skidding is undesirable in that the vehicle stopping distance can be lengthened and vehicle control is reduced. In a typical ABS, a wheel speed sensor senses when individual wheels on a vehicle begin to "lock-up" (i.e., cease rotation) during braking, which is an indication that those vehicle wheels are skidding. Accordingly, in order to minimize such skidding, the ABS modulates hydraulic fluid flow to the locked wheel brake cylinder, thereby causing the brake to cycle between apply and release modes. The modulation of hydraulic fluid flow to a locking wheel brake prevents wheel lock-up while continuing to apply braking pressure, thereby allowing the driver of the associated vehicle to stop the vehicle in a safer, more controlled manner.
The modulation of fluid flow to the wheel brake cylinders can be accomplished in a variety of ways, but in standard motor-based ABS's, the hydraulic fluid modulation is effected by the controlled manipulation of ABS apply and release valves upstream of the brake cylinder with the assistance of hydraulic fluid pumps. The apply and release valves are positioned in a bypass loop and are operated such that when an ABS braking event is detected at one of the wheels, pressure initially is bled from the wheels back to the master cylinder until wheel lock-up is overcome. Then, as additional braking force is needed, the apply and release valves "pulse," thereby providing braking force to the wheel as desired while preventing wheel lock-up and undesirable skidding. Accordingly, in this manner hydraulic fluid pressure is modulated until the vehicle reaches an acceptably safe speed, such as 5 kph. At this time, in order to return braking control to the driver, the ABS release valves are closed and the apply valves are pulsed, thereby gradually restoring full master cylinder output to the wheel brakes.
The above-mentioned pumps are used in the ABS to return hydraulic fluid to the master cylinder during the ABS event. However, since the pumps are working against constant pressure being applied by the driver during the ABS event, the pumps usually cannot return the fluid to the master cylinder as quickly as the fluid is being delivered to the pumps. Accordingly, overflow reservoirs, such as hydraulic accumulators, are positioned between the vehicle wheels and the pumps to collect excess fluid that is delivered to the pumps during the ABS event. The collection of the fluid prevents back pressure that would defeat the ABS operation from being transmitted to the wheel. Then, as the pumps are able to catch up, fluid is evacuated from the accumulators and pumped back to the master cylinder. This clearing of the accumulators is important to prepare the ABS for the next ABS braking event.
Traditionally, the accumulator clearing routine, known as pump "run on," has been accomplished simultaneously with the pulsing of the ABS apply valves returning driver master cylinder brake control, thereby masking some of the pump motor noise. However, since the accumulator clearing routine generally requires up to 700 ms or more of pump motor run time to ensure adequate draining of the accumulators, and ABS apply valve pulsing typically takes only 300 ms, the pump motor noise is not masked during the last 400 ms or so of the pump motor run time. Thus the running of the pump motor after the end of the ABS apply valve pulse routine is not masked. This unmasked noise is considered undesirable because it can be loud and disturbing to vehicle occupants. The noise is particularly objectionable during low coefficient of friction stops when there is very little road noise to cover the ABS pump motor operating noise.
Furthermore, since the pump motors are typically being actuated while the vehicle is decelerating after the 5 kph "safe" speed has been reached, the driver is usually still applying pressure to the brake pedal, thereby building up significant pressure in the master cylinder. This pressure further exacerbates the noise problem in that the load on the ABS pump motor is increased, adding significantly to the noise created by the motor as well as increasing the time required to clear the accumulators fully. Additionally, operating the motor for the pumps against the high pressure is detrimental to the life of the motor.
Thus, given the disadvantages of prior art ABS pump motor control routines, there is a need for a control routine for ABS pump motors that reduces the amount of objectionable noise experienced by the occupants of an automotive vehicle when the pump motor is in operation, that lessens the stress placed on the pump motor and that satisfactorily clears a hydraulic reservoir, such as an accumulator, of stored fluid.